1. Field of the Invention
The present invention relates to a single mode optical waveguide, more particularly relates to an optical waveguide for use as an optical modulator and an optical switch in an optical integrated circuit used in an optical fiber communication system, an optical information processing system, an optical sensing system, an optical data registration system and the like.
2. Description of the Related Art
Recently, due to progresses of a single mode optical fiber and a single wavelength laser, a high speed optical data transmission of G b/s has practically been utilized. In particular, the optical waveguide device, which is superior in a high adaptability with the single mode optical fiber, holds a key to a future development of the optical fiber communication system.
In the field of the optical communication system, in order to build up the optical fiber communication networks, it is necessary to develop the optical device such as an optical matrix switch, the optical modulator or the like. The single mode optical waveguide is a component of such an optical device.
As an optical device, hithertofore, a number of optical devices using a Ti diffused LiNbO.sub.3 channel waveguide have been developed. LiNbO.sub.3 is generally utilized as an optical device using the electro-optical effect, since it has a relatively larger electro-optical constant among stable inorganic optical crystals. A Ti diffusion method and a proton exchange method are well known as a method of forming a waveguide.
Such an optical device is constructed by utilizing a single mode channel waveguide capable of transmitting only a fundamental mode in order to prevent unnecessary mode conversion due to interference and little disturbance between modes in waveguide.
As a material for constructing the optical device, as described above, LiNbO.sub.3 is desirable and actually used. However, in such a LiNbO.sub.3, (1) a LiNbO.sub.3 optical waveguide produced by the Ti diffusion method can not perform waveguiding of a short wave length or a visible light due to its a large optical damage, and (2) a LiNbO.sub.3 optical waveguide produced by the proton exchange method has a different crystallizability from that of a virgin LiNbO.sub.3, after the waveguide has been formed, so that together with the other reason, an electro-optic constant is smaller than that of a bulk crystal (or LiNbO.sub.3). Accordingly, when such an optical waveguide is used as the optical device for an optical switch or a directional coupler and the optical modulator or the like, then problem has arisen in an inability to realize the electric-power-saving and the downsizing since a larger switching voltage and a longer effective length are required.
While, the Ti diffused LiNbO.sub.3 waveguide is formed by diffusing Ti at 1000.degree. C., and the proton exchanged LiNbO.sub.3 waveguide is also formed by performing the proton exchange at about 200.degree. C. Therefore, when the temperature arises up to the Curie point of the LiNbO.sub.3 for these waveguides, there is a problem that the Ti or Proton are more diffused enough to change a waveguide mode profile or to eliminate a presence of the waveguide mode.
Then, the present inventors have found that as shown in Japanese Patent Application Opened No. 12,095/92, the above problem is caused by a lower electro-optic effect caused from lattice mismatch between a substrate and a crystalline material forming the waveguide. In order to solve the above problem, the lattice match between the LiTaO.sub.3 monocrystalline substrate and the LiNbO.sub.3 monocrystalline thin film waveguide layer is performed by including an Na content in a range of 0.1 to 14.3 mol % and an Mg content in a range of 0.8 to 10.8 mol % in the LiNbO.sub.3 monocrystalline waveguide, thereby developing a method of producing an optical device with a superior electro-optic effect.
In order to use such an optical waveguide as a practical device, as described above, the single mode optical waveguide must be obtained, but the above lattice matched LiNbO.sub.3 waveguide developed by the present inventors has a different refractive index from that of the conventional LiNbO.sub.3 waveguide so that conditions for single mode are different with each other and thus the LiNbO.sub.3 waveguide obtained by the conventional design did not exhibit a single mode transmission.